A filter (2) for filtering mutagens from an engine oil, which filter (2) comprises a body (4), an inlet (12) for oil passing into the body (4), an outlet (14) for the oil passing out of the body (4), and a filter (2) material in the body (4) for filtering the oil as the oil passes through the body (4), and the filter (2) material being such that it comprises activated carbon (16) for removing the mutagens.